An insulating material subjected to the combined action of an electrical gradient and external agents (moisture, solar radiation, pollutants, etc.) is subject to degradation as a result of the phenomenon known as "tracking". This is the situation affecting the material forming the outer sheath of an optical fibre cable suspended near a high-voltage overhead line. The gradient applied is the difference between the potential of the electric field around the cable and the earth, to which the cable is connected by the clamps fixing it to the supporting pylons. In the proximity of these clamps, the insulating material is subject to the maximum variation of the electric field.
When the cable is dry, the high surface resistance of the sheath prevents the flow of current, and consequently there is no evidence of degradation. Exposure to atmospheric agents, and to solar radiation in particular, causes a surface oxidation of the material which increases its affinity for water. When the sheath starts to retain water, and with it the atmospheric pollutants, the surface resistance decreases drastically and current therefore flows through the film of water coating the sheath. This flow of current produces, by the Joule effect, a local heating of the material, with evaporation of the water. The situation becomes critical when the sheath is only partially wet, so that moist areas with low electrical resistance alternate with dry areas with much higher resistance. The high conductivity gradient at the points of separation between dry and moist areas leads to the formation of electrical discharges which intensely overheat the underlying sheath, with consequent degradation of the polymer material and formation of paths (tracking) and true areas of erosion which extend rapidly until the rupture of the material is caused. In the case of polyolefin materials, the degradation is manifested by an initial melting followed by oxidation, with a consequent increase in local wettability and therefore of the number of electrical discharges, such that the polymer is caused to ignite.
The installation of self-supporting optical cables made of dielectric material in the vicinity of high-voltage (usually from 150 to 380 kV) overhead lines therefore requires the use of sheaths with high resistance to the said tracking phenomena, which at the same time have good mechanical properties and low density, so that light cables resistant to mechanical stresses can be produced.